If California is as serious about public transit as its leaders claim, why isn’t there rail service across the Golden Gate Bridge?

There’s no good reason why our state’s iconic span must devote its six lanes to cars. For more than 50 years, engineering studies have shown that the bridge could accommodate trains.

Today, there is no more glaring hole in California public transportation than the one across the Golden Gate Bridge.

North of the bridge, Sonoma and Marin counties are about to open the first phase, from Santa Rosa to San Rafael, of their SMART light-rail service. SMART, which includes a bicycle-pedestrian pathway, will eventually serve a 70-mile corridor from Cloverdale to Larkspur, just 10 miles up Highway 101 from the Golden Gate.

South of the bridge, San Francisco is spending billions to construct the Transbay Transit Center, a Grand Central Station of the West. Eventually, it is supposed to be the northern terminus of high-speed rail.

But there is no plan for a train to connect the new SMART train with the new giant transit station. That’s a shocking failure for Bay Area do-gooders, who love to lecture the rest of us on the need to go boldly into the future. What in the name of progressive enlightenment are you waiting for?

The idea of a Golden Gate Bridge train is not new. Such service was envisioned as part of the original plan for the BART system. Michael C. Healy, in his excellent new book “BART: The Dramatic History of the Bay Area Rapid Transit System,” recalls that Marin County leaders in the early 1960s badly wanted to be part of BART.

But in 1961, the governing authority of the Golden Gate Bridge balked at allowing trains, claiming that they would put too much stress on support cables. BART’s own engineering studies found that the bridge was plenty strong enough, but the bridge authority wouldn’t budge. In the end, BART dropped Marin from its plans, to the frustration of several county officials.

The dream didn’t die. In 1990, renewed talk of BART to Marin led to a study that found the bridge could handle trains. But the multibillion-dollar cost of taking BART to the North Bay ended the conversation.

In this history, there’s a lesson even more dramatic than the Golden Gate: There are huge costs when California skimps on infrastructure. A bridge train to the North Bay would have been easier and cheaper in the 1970s than now, and so for 40 years North Bay commuters have paid a rapidly rising price — in traffic, in tolls, time, and in the extortionate cost of parking in San Francisco.

There is real wisdom in the phrase that Healy attributes to Bill Stokes, the founding father of BART: “Build it now. It will never be cheaper.”

That’s why a train link over the Golden Gate Bridge would still make sense today. Yes, such a plan would be attacked — this is the Bay Area and this is California, after all. Preservationists would say an iconic American landmark is being sullied. Marin’s antigrowth zealots would argue that a train would encourage new development in their idylls. Pointy-headed accounting types would cite the cost and point out that most commuters in the North Bay are going to jobs in the North Bay, not in the city.

And those who follow BART closely will argue that the system is at a difficult crossroads, and needs to focus on maintenance and other pressing projects, like a second tube under the bay between Oakland and San Francisco.

To all such objections there is one answer: Why is the Bay Area thinking so narrowly and with so little vision for the future? As an Angeleno, I can’t resist pointing out to Bay Area friends that in the realm of public transit, we in Southern California are surpassing you, having passed sales-tax increases to fund a transformational 50-year plan for a regional system that makes yours look like a disjointed joke. Are you really going to let yourself be embarrassed by L.A.?

Imagine how powerful a symbol of California’s connected future a Golden Gate Bridge-traversing train would be. It would draw commuters and tourists alike, making the planet’s greatest bridge even greater.

Such a train could be the inspirational showpiece for what the Bay Area badly needs: a new regional plan for transit that connects all nine of its counties. You’ve come to this bridge, California. It’s time to cross it.